"Long before it's in the papers"
January 27, 2015


Fifth Pluto moon found

July 12, 2012
Courtesy of the Space 
Telescope Science Institute
and World Science staff

As­tro­no­mers us­ing NASA’s Hub­ble Space Tel­e­scope are re­port­ing the dis­cov­ery of a fifth moon or­bit­ing the icy “d­warf plan­et” Plu­to.

The ob­ject is thought to be ir­reg­u­larly shaped, six to 15 miles (10 to 24 km) wide and to trav­el around Plu­to in a 58,000-mile- (93,000-km-) wide cir­cu­lar or­bit, a path thought to lie in the same plane as that of the oth­er moons. They “form a se­ries of neatly nest­ed or­bits, a bit like Rus­sian dolls,” said re­search team leader Mark Showal­ter of the SETI In­sti­tute in Moun­tain View, Ca­lif.

This im­age, tak­en by NA­SA's Hub­ble Space Tel­e­scope, shows five moons or­bit­ing the dis­tant, icy dwarf plan­et Plu­to. The green cir­cle marks the new­ly dis­cov­ered moon, des­ig­nat­ed P5, as pho­tographed by Hub­ble's Wide Field Cam­era 3 on Ju­ly 7. Oth­er ob­ser­va­tions that col­lec­tive­ly show the moon's or­bit­al mo­tion were tak­en on June 26, 27, 29, and Ju­ly 9, 2012. The moon is es­ti­mat­ed to be 6 to 15 miles across. It is in a 58,000-mile-diameter cir­cu­lar or­bit around Plu­to that is as­sumed to be co-planar with the oth­er satel­lites in the sys­tem. The ob­ser­va­tions are ex­pected to help sci­en­tists in their plan­ning for the Ju­ly 2015 fly­by of Plu­to by NA­SA's New Hori­zons space­craft. (Cred­it: NA­SA, ESA, and M. Showal­ter (SETI In­sti­tute))

The sci­en­tists called it in­tri­guing that such a small plan­et can have such a com­plex col­lec­tion of satel­lites. 

The find­ing al­so pro­vides new clues to how the Plu­to sys­tem formed and evolved. The fa­vored the­o­ry is that all the moons are relics of a billions-of-years-old col­li­sion be­tween Plu­to and some­thing else in its ar­ea of or­bit, called the Kui­per belt.

The new finding is ex­pected to help sci­en­tists nav­i­gate NASA’s New Hori­zons space­craft through the Plu­to sys­tem in 2015, when it makes an his­tor­ic and long-a­wait­ed high-speed fly­by of the dis­tant world. 

The team is us­ing Hub­ble’s pow­er­ful vi­sion to scour the Plu­to sys­tem to un­cov­er po­ten­tial haz­ards to the New Hori­zons space­craft. Mov­ing past Plu­to at 30,000 miles (50,000 km) per hour, New Hori­zons could be de­stroyed in a col­li­sion with even a BB-shot-size piece of or­bital de­bris.

“The dis­cov­ery of so many small moons in­di­rectly tells us that there must be lots of small par­t­i­cles lurk­ing un­seen in the Plu­to sys­tem,” said Har­old Weav­er of the Johns Hop­kins Uni­vers­ity Ap­plied Phys­ics Lab­o­r­a­to­ry in Lau­rel, Md. “The invento­ry of the Plu­to sys­tem we’re tak­ing now with Hub­ble will help the New Hori­zons team de­sign a safer trajecto­ry for the space­craft,” added Al­an Stern of the South­west Re­search In­sti­tute in Boul­der, Co­lo., the mis­sion’s prin­ci­pal in­ves­ti­ga­tor.

Plu­to’s larg­est moon, Char­on, was discov­ered in 1978 in ob­serva­t­ions made at the U.S. Na­val Observato­ry in Wash­ing­ton, D.C. Hub­ble ob­serva­t­ions in 2006 un­cov­ered two ad­di­tion­al small moons, Nix and Hy­dra. In 2011 anoth­er moon, P4, was found in Hub­ble da­ta. The lat­est moon, provi­sionally de­signated S/2012 (134340) 1, was de­tected in nine sets of im­ages tak­en by Hub­ble’s Wide Field Cam­era 3 this month and last month. 

In the years fol­low­ing the New Hori­zons Plu­to fly­by, as­tro­no­mers plan to use the in­fra­red vi­sion of Hub­ble’s planned suc­ces­sor, NASA’s James Webb Space Tel­e­scope, for fol­low-up ob­serva­t­ions. The Webb tel­e­scope is ex­pected to be able to meas­ure Plu­to’s sur­face chem­is­try, its moons, and many oth­er bod­ies that lie in the dis­tant Kuipe­r Belt along with Plu­to.

* * *

Send us a comment on this story, or send it to a friend


Sign up for

On Home Page         


  • St­ar found to have lit­tle plan­ets over twice as old as our own

  • “Kind­ness curricu­lum” may bo­ost suc­cess in pre­schoolers


  • Smart­er mice with a “hum­anized” gene?

  • Was black­mail essen­tial for marr­iage to evolve?

  • Plu­to has even cold­er “twin” of sim­ilar size, studies find

  • Could simple an­ger have taught people to coop­erate?


  • F­rog said to de­scribe its home through song

  • Even r­ats will lend a help­ing paw: study

  • D­rug may undo aging-assoc­iated brain changes in ani­mals

Astronomers using NASA’s Hubble Space Telescope are reporting the discovery of a fifth moon orbiting the icy “dwarf planet” Pluto. The object is estimated to be irregularly shaped and 6 to 15 miles (10 to 24 km) wide and to travel around Pluto in a 58,000-mile- (93,000-km-) wide circular orbit, a path thought to lie in the same plane as that of the other moons. They “form a series of neatly nested orbits, a bit like Russian dolls,” said resarch team lead Mark Showalter of the SETI Institute in Mountain View, Calif. The scientists called it intriguing that such a small planet can have such a complex collection of satellites. The finding also provides new clues to how the Pluto system formed and evolved. The favored theory is that all the moons are relics of a billions-of-years-old collision between Pluto and something else in its area of orbit, called the Kuiper belt. The discovery is expected to help scientists navigate NASA’s New Horizons spacecraft through the Pluto system in 2015, when it makes an historic and long-awaited high-speed flyby of the distant world. The team is using Hubble’s powerful vision to scour the Pluto system to uncover potential hazards to the New Horizons spacecraft. Moving past Pluto at 30,000 miles (50,000 km) per hour, New Horizons could be destroyed in a collision with even a BB-shot-size piece of orbital debris. “The discovery of so many small moons indirectly tells us that there must be lots of small particles lurking unseen in the Pluto system,” said Harold Weaver of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. “The inventory of the Pluto system we’re taking now with Hubble will help the New Horizons team design a safer trajectory for the spacecraft,” added Alan Stern of the Southwest Research Institute in Boulder, Colo., the mission’s principal investigator. Pluto’s largest moon, Charon, was discovered in 1978 in observations made at the U.S. Naval Observatory in Washington, D.C. Hubble observations in 2006 uncovered two additional small moons, Nix and Hydra. In 2011 another moon, P4, was found in Hubble data. The latest moon, provisionally designated S/2012 (134340) 1, was detected in nine sets of images taken by Hubble’s Wide Field Camera 3 this month and last month. In the years following the New Horizons Pluto flyby, astronomers plan to use the infrared vision of Hubble’s planned successor, NASA’s James Webb Space Telescope, for follow-up observations. The Webb telescope is expected to be able to measure Pluto’s surface chemistry, its moons, and many other bodies that lie in the distant Kuiper Belt along with Pluto.